1 00:00:03,350 --> 00:00:06,950 This lesson you'll learn how to use functions in your scripts. 2 00:00:06,990 --> 00:00:13,350 First off a function is simply a group of commands that you call using a single name in your script. 3 00:00:13,470 --> 00:00:18,120 You can think of functions as being little scripts inside of your main script. 4 00:00:18,180 --> 00:00:24,980 You can call or execute a function just like you would any other script or command on a Linux system. 5 00:00:25,260 --> 00:00:30,180 There are a couple of reasons why you would want to use a functions. 6 00:00:30,180 --> 00:00:37,470 The first reason is that you don't want to have duplicate code in multiple places in your script using 7 00:00:37,500 --> 00:00:41,380 functions allows your code to remain dry dry. 8 00:00:41,380 --> 00:00:46,130 Is a computer programming an application development principle that stands for. 9 00:00:46,170 --> 00:00:47,910 Don't repeat yourself. 10 00:00:48,000 --> 00:00:54,060 Functions allow you to write a block of code once and use it many times instead of repeating several 11 00:00:54,060 --> 00:00:59,670 lines of code each time you need to perform a particular task simply call the function that contains 12 00:00:59,670 --> 00:01:00,690 that code. 13 00:01:00,690 --> 00:01:07,470 This helps in reducing the length of your scripts and also gives you a single place to change test troubleshoot 14 00:01:07,500 --> 00:01:09,950 and document a given task. 15 00:01:09,960 --> 00:01:13,380 All of this makes your scripts easier to maintain. 16 00:01:14,350 --> 00:01:21,010 By the way there is a counter principle called wet which stands for write everything twice or we enjoy 17 00:01:21,010 --> 00:01:24,220 typing or waste everyone's time. 18 00:01:24,220 --> 00:01:30,520 If you catch yourself typing out this same bit of code or you find yourself copying and pasting existing 19 00:01:30,520 --> 00:01:35,680 code in your script it's time to dry up that wet code and use a function. 20 00:01:35,680 --> 00:01:42,530 The second reason to use functions is to break up large tasks into a series of smaller tasks. 21 00:01:42,730 --> 00:01:45,600 This makes your code easier to maintain. 22 00:01:45,610 --> 00:01:51,580 You can zoom into a function that only does one small thing and make your changes around that small 23 00:01:51,580 --> 00:01:52,550 thing there. 24 00:01:52,600 --> 00:01:54,880 And that one place. 25 00:01:54,880 --> 00:02:00,850 For example if you are using a case statement to make a decision then instead of writing a whole long 26 00:02:00,850 --> 00:02:06,870 block of code inside the case statement you could call a function that actually does the real work. 27 00:02:06,880 --> 00:02:08,740 All right so enough philosophizing. 28 00:02:08,740 --> 00:02:12,040 Let's get to work and start creating some functions. 29 00:02:12,040 --> 00:02:14,900 So first off here as always I have my terminal open. 30 00:02:14,980 --> 00:02:18,490 I'm going to change into our class folder of shell class. 31 00:02:18,490 --> 00:02:20,530 We're still working on the locals. 32 00:02:20,530 --> 00:02:22,380 Users of vagrant project. 33 00:02:22,600 --> 00:02:37,030 And now I'm going to bring up this VM with vagrant up. 34 00:02:37,160 --> 00:02:40,960 One way to create a function is to use the function built in command. 35 00:02:42,980 --> 00:02:44,680 Here you start with the word function. 36 00:02:44,690 --> 00:02:50,440 Give your function a name then surround a series of commands inside curly braces. 37 00:02:50,600 --> 00:02:54,470 The other way you can create a function is to give your function a name. 38 00:02:54,590 --> 00:03:00,440 Follow it with a set of parentheses then surround a series of commands inside curly braces. 39 00:03:00,440 --> 00:03:03,200 Now let's jump into a script and try it out. 40 00:03:06,310 --> 00:03:11,520 Let's create a function called log that will end up logging messages to the system log and optionally 41 00:03:11,530 --> 00:03:18,100 displaying those messages to the user executing the script will iterate over this bit of code and keep 42 00:03:18,100 --> 00:03:25,470 going as we go but we'll start out here will create a function by providing the name. 43 00:03:25,520 --> 00:03:30,260 Then following that name with parentheses and then we're going to create a block of code surrounded 44 00:03:30,260 --> 00:03:33,600 in curly braces here and we'll just do this for now. 45 00:03:33,600 --> 00:03:41,790 Echo you call the log function and end it with a closing curly brace. 46 00:03:41,880 --> 00:03:48,420 Now a color will just use the word log and that will execute it just like any other command on a Linux 47 00:03:48,420 --> 00:03:49,340 system. 48 00:03:49,350 --> 00:03:54,360 Now remember that scripts get read and executed from the top down. 49 00:03:54,360 --> 00:03:57,420 That means you have to define a function before you can use it. 50 00:03:57,420 --> 00:04:01,460 That's why we define the log of function at the top of your script here. 51 00:04:01,560 --> 00:04:08,430 When a function is defined it is read into memory and it's then available for use. 52 00:04:08,490 --> 00:04:15,020 The code in the function is just remembered initially and it's only executed when it gets called. 53 00:04:15,030 --> 00:04:19,290 Notice that when we called the function we didn't use the parentheses. 54 00:04:19,320 --> 00:04:24,000 You may have seen this type of syntax and style and other programming languages where you actually use 55 00:04:24,000 --> 00:04:27,360 the parentheses but it doesn't work in shell scripts. 56 00:04:27,360 --> 00:04:32,310 Simply place the name of a function on a line and it will execute that function. 57 00:04:32,310 --> 00:04:33,760 So let's try it out. 58 00:04:38,380 --> 00:04:43,180 Hey sure enough we get the output that says you call the log function. 59 00:04:43,630 --> 00:04:47,950 Hey that's not very interesting yet but we'll get there soon enough so let's get back into our script 60 00:04:47,950 --> 00:04:48,940 here. 61 00:04:48,940 --> 00:04:54,220 I want to show you the other way you can define a function so how you can do that is use the keyword 62 00:04:54,220 --> 00:04:55,130 function. 63 00:04:55,390 --> 00:04:57,620 All that by your function name. 64 00:04:57,740 --> 00:05:03,220 And then you don't need the parentheses here and will save our changes and test this as well. 65 00:05:03,460 --> 00:05:07,490 So as you can see it also works so either method works. 66 00:05:07,510 --> 00:05:11,340 I'm going to go back and use this particular style here. 67 00:05:12,330 --> 00:05:16,370 Now let's make our log function display to the screen whatever is passed to it. 68 00:05:16,560 --> 00:05:20,050 So I'll type out the code here and then we'll talk through it in just a second. 69 00:05:20,310 --> 00:05:22,680 So what we're going to do here is this. 70 00:05:52,910 --> 00:06:00,140 I've introduced a new show built in called Local what local does is makes a variable local in scope 71 00:06:00,200 --> 00:06:01,490 to the function. 72 00:06:01,490 --> 00:06:07,280 That means the value of that variable is only accessible inside that function. 73 00:06:07,280 --> 00:06:13,490 So in this example if you tried to display the contents of the message a variable before or after the 74 00:06:13,490 --> 00:06:16,460 function it would be blank and unset. 75 00:06:16,490 --> 00:06:20,360 That's because it only exists inside the function. 76 00:06:20,390 --> 00:06:25,860 By the way the local command can only be used inside a function that's the only place it makes sense. 77 00:06:26,000 --> 00:06:32,750 And the reason why is that until now we've been using global variables and that means that the variable 78 00:06:32,750 --> 00:06:39,770 and its contents or its value can be accessed anywhere in the script including in any function. 79 00:06:39,770 --> 00:06:44,090 Now we haven't been using functions until now so it really didn't matter until this point. 80 00:06:44,210 --> 00:06:50,450 By the way if you define a global variable in a function by omitting the local keyword in front of it 81 00:06:50,780 --> 00:06:57,290 that global variable is not available outside that function until the function is called and executed 82 00:06:57,760 --> 00:07:01,780 it's not a best practice to do that anyway so it shouldn't really be an issue. 83 00:07:01,970 --> 00:07:07,910 However when you are using functions just be aware of the scope of each variable and know that it can 84 00:07:07,910 --> 00:07:12,410 be a place that can deserve some attention when troubleshooting your scripts. 85 00:07:12,470 --> 00:07:16,520 It's a best practice to use local variables inside your functions. 86 00:07:16,520 --> 00:07:22,370 That way you won't accidentally reuse the same variable name in another function and end up with a tricky 87 00:07:22,370 --> 00:07:23,090 bug. 88 00:07:23,090 --> 00:07:28,160 However you'll find plenty of shell scripts that do not add here to this recommendation. 89 00:07:28,190 --> 00:07:30,440 Most of the time it will not cause a problem. 90 00:07:30,440 --> 00:07:35,210 Just be sure to use unique variable names throughout your script and you'll be fine. 91 00:07:35,210 --> 00:07:40,710 Now let's talk about the value that is assigned to the local variable named message. 92 00:07:40,730 --> 00:07:46,280 It's our old friend dollar sign and sign back from our positional parameters lesson as you recall a 93 00:07:46,280 --> 00:07:53,270 dollar sign at sign expands to all the positional parameters starting from one functions act like a 94 00:07:53,270 --> 00:07:59,000 script within your script and you can accept arguments which can be then accessed as positional parameters 95 00:07:59,060 --> 00:08:00,730 inside the function. 96 00:08:00,830 --> 00:08:04,920 For example the first argument passed to a function is dollar sign one. 97 00:08:04,940 --> 00:08:07,750 The second is Dollar Sign 2 and so on. 98 00:08:07,760 --> 00:08:13,850 The only difference between a function and a script here is that dollar sign is zero is still the name 99 00:08:13,850 --> 00:08:17,640 of the shellscript itself and not the name of the function. 100 00:08:17,660 --> 00:08:20,650 Even if you're using dollar signs zero in the function. 101 00:08:20,840 --> 00:08:26,210 And that's really OK anyway because you'd never really want to use that functionality anyway. 102 00:08:26,570 --> 00:08:28,620 OK let's try this bit of code out. 103 00:08:31,240 --> 00:08:36,880 When we execute our script the function is called first time with Hello it print the screen the second 104 00:08:36,880 --> 00:08:38,650 time the function is called with. 105 00:08:38,650 --> 00:08:42,710 This is fun and that also gets displayed to the screen. 106 00:08:43,240 --> 00:08:47,720 Now let's update our function to only display the message that was passed to it. 107 00:08:47,770 --> 00:08:52,110 If a global variable named verbose is set to true 108 00:09:02,740 --> 00:09:11,090 so will check to see if both is true and if it is then will echo that message to the screen and the 109 00:09:11,090 --> 00:09:14,090 ephi there closes out our IF statement. 110 00:09:14,150 --> 00:09:21,650 So let's do this let's call our function log with hello and then let's go ahead and now set verbose 111 00:09:22,040 --> 00:09:23,230 equal to true. 112 00:09:23,240 --> 00:09:28,010 So the first time we call a function rebozo is not set the second time we call it. 113 00:09:28,070 --> 00:09:30,270 It is set to true. 114 00:09:30,380 --> 00:09:36,860 Now this function is actually starting to become useful if we didn't have this function then we would 115 00:09:36,860 --> 00:09:43,250 have to test for the value of the verbose variable every single time we needed to decide if we wanted 116 00:09:43,250 --> 00:09:45,530 to display something to the screen or not. 117 00:09:45,530 --> 00:09:50,170 This means we would end up with a lot of duplicate code throughout our script. 118 00:09:50,180 --> 00:09:56,180 In any case what should happen is that the text below will not be displayed to the screen because verbose 119 00:09:56,180 --> 00:10:01,970 has not been set to true at that point and the text this is Vonne will be displayed because as I just 120 00:10:01,970 --> 00:10:08,460 explained verbose is set to true before log is called with that particular text passed into it. 121 00:10:08,460 --> 00:10:10,620 So let's try that out. 122 00:10:13,730 --> 00:10:18,670 Hey sure enough Hello was not displayed to the screen but this is fun. 123 00:10:18,690 --> 00:10:20,710 Was just as we expected. 124 00:10:21,780 --> 00:10:27,150 If you've ever had any formal training in computer programming you've probably heard that global variables 125 00:10:27,150 --> 00:10:30,650 are evil and you should avoid them at all costs. 126 00:10:30,660 --> 00:10:36,060 Really what you should try to avoid at all costs are the problems that can arise by using global variables 127 00:10:36,060 --> 00:10:40,710 such as different functions changing the value of a global variable and so on. 128 00:10:40,710 --> 00:10:46,020 Now I'm a pretty pragmatic person so I like to do what is best for a given situation. 129 00:10:46,020 --> 00:10:52,410 In that vein let me give you an example of why avoiding global variables can be less than ideal especially 130 00:10:52,410 --> 00:10:54,850 with our little use case here. 131 00:10:54,900 --> 00:11:01,410 We could change this function such that it doesn't rely on the verbose global variable and instead make 132 00:11:01,410 --> 00:11:04,830 it an argument that has to be passed into the function. 133 00:11:04,830 --> 00:11:06,420 So what we can do is this. 134 00:11:06,420 --> 00:11:14,310 We can add local verb Bowse and set that to the value that is passed into it here. 135 00:11:14,380 --> 00:11:21,070 The first value that's passed into it then we'll use shift here and then at that point every thing else 136 00:11:21,070 --> 00:11:22,350 stays the same. 137 00:11:22,360 --> 00:11:28,100 So what this does is assigns the first value passed into the function into the local variable named 138 00:11:28,140 --> 00:11:29,090 verbose. 139 00:11:29,140 --> 00:11:34,490 Then the shift command shifts all those positional parameters which removes the value stored in Dallas 140 00:11:34,500 --> 00:11:36,870 and one makes dollar sign to dollar sign. 141 00:11:36,870 --> 00:11:40,120 One makes the dollar sign three dollar sign too and so on. 142 00:11:40,120 --> 00:11:46,930 So more or less we chop off the value we assigned to verbose and what we're left with is the message 143 00:11:46,960 --> 00:11:49,210 that was passed into the function. 144 00:11:49,210 --> 00:11:52,120 Now we have to update how we call this function. 145 00:11:52,180 --> 00:11:59,350 So here we're going to do log true to tell it that we're going to be verbose and we can get rid of this 146 00:11:59,350 --> 00:12:00,650 piece of code here. 147 00:12:00,880 --> 00:12:05,330 And we're also going to do this we'll say true here as well. 148 00:12:05,380 --> 00:12:11,350 Now every time we use the log function we need to tell it if we want verbose to be true or false. 149 00:12:11,380 --> 00:12:18,520 Of course we could set a variable and use that like this so we can do this we could say verbosity is 150 00:12:18,520 --> 00:12:22,300 true and then we'll do this 151 00:12:31,720 --> 00:12:32,700 OK. 152 00:12:33,160 --> 00:12:38,400 In practice the verbosity is going to remain the same for our entire script because of that. 153 00:12:38,410 --> 00:12:43,930 I could argue that passing it into the function every single time is causing us to repeat ourselves 154 00:12:43,930 --> 00:12:46,850 without any real practical gain. 155 00:12:46,870 --> 00:12:49,960 In this example I think using a global variable is fine. 156 00:12:49,960 --> 00:12:53,830 However if you disagree with me that's also fine in my opinion. 157 00:12:53,830 --> 00:12:55,670 Both methods are correct. 158 00:12:55,720 --> 00:12:59,250 So long as they get the job done successfully. 159 00:12:59,260 --> 00:13:04,480 Now let's just show that this little piece of code works so we'll see where changes and execute our 160 00:13:04,480 --> 00:13:13,530 script and sure enough it executes the function and displays what we passed to it to the screen if we 161 00:13:13,530 --> 00:13:18,690 were doing something like differentiating informational messages from morning messages from critical 162 00:13:18,690 --> 00:13:24,950 messages then passing in that log level would make sense because it can change throughout her script. 163 00:13:24,960 --> 00:13:27,750 So again use what works and what makes sense to you. 164 00:13:27,750 --> 00:13:29,490 Just make sure that it actually works. 165 00:13:29,490 --> 00:13:31,170 Test your scripts. 166 00:13:31,230 --> 00:13:36,690 Now let's revert our code and handle the main concern of using global variables within functions which 167 00:13:36,690 --> 00:13:41,430 is that a function might actually change the value of that global variable. 168 00:13:41,610 --> 00:13:42,500 So we'll do this 169 00:13:59,340 --> 00:14:04,950 the read only shell builtin makes a variable unchangeable or an modifiable. 170 00:14:04,950 --> 00:14:10,500 This is the shells version of a constant variable which is a term you might be familiar with if you've 171 00:14:10,500 --> 00:14:12,230 programmed in other languages. 172 00:14:12,540 --> 00:14:15,620 There are two ways to use the read only shell built in. 173 00:14:15,630 --> 00:14:20,150 One is just like we have it here on the screen which is to use the word read only. 174 00:14:20,160 --> 00:14:25,520 Followed by the variable and then include the variable assignment there right after it. 175 00:14:25,530 --> 00:14:31,350 The other way is to perform the variable assignment first and then use Read-Only followed by just the 176 00:14:31,350 --> 00:14:32,420 variable name. 177 00:14:32,490 --> 00:14:36,770 There's no need to include the assignment because it's already performed at that point. 178 00:14:36,780 --> 00:14:42,280 This way you can separate the setting of the variable and marking it as read only. 179 00:14:42,570 --> 00:14:47,640 Now that the verbose variable is read only it cannot be changed for the duration of the script. 180 00:14:47,760 --> 00:14:51,610 That means it cannot be changed inside or outside of a function. 181 00:14:51,690 --> 00:14:56,270 That eliminates a concern about a function changing the value of a global variable. 182 00:14:56,610 --> 00:15:01,920 OK we've taken care of the first bit of our function that controls whether or not we display a message 183 00:15:01,920 --> 00:15:03,110 to standard output. 184 00:15:03,270 --> 00:15:08,700 Now let's write the last little piece that will control sending messages to the system's log. 185 00:15:08,700 --> 00:15:12,150 To do that we'll use a command line utility named logger. 186 00:15:12,150 --> 00:15:15,270 Let's actually take a look at that on the command line. 187 00:15:19,470 --> 00:15:19,850 OK. 188 00:15:19,880 --> 00:15:26,010 Logger is a executable so we can run man against that to get some information about that particular 189 00:15:26,010 --> 00:15:26,720 command. 190 00:15:27,700 --> 00:15:34,060 Mainly you just supply logger a message and that message is recorded and the systems log by default 191 00:15:34,060 --> 00:15:35,920 on a Sentosa rail system. 192 00:15:35,920 --> 00:15:39,910 Those messages will go to the var log messages file. 193 00:15:39,910 --> 00:15:44,680 You can read all the details here if you like but I'm just going to point out the one bit of information 194 00:15:44,680 --> 00:15:51,940 we're going to use most immediately and that is the dash t option which allows you to tag your Ciss 195 00:15:51,970 --> 00:16:00,720 log message on the move down here to Dashti this tag is typically set to the name of your program or 196 00:16:00,720 --> 00:16:03,880 the program that is writing to the system log file. 197 00:16:03,960 --> 00:16:05,430 So let's try that out now. 198 00:16:05,460 --> 00:16:08,870 Q To get out of this help and we'll do this longer. 199 00:16:10,290 --> 00:16:13,660 From the command line. 200 00:16:13,740 --> 00:16:14,100 All right. 201 00:16:14,100 --> 00:16:20,610 And we want to look at the var log messages file and that file happens to have permission such that 202 00:16:20,610 --> 00:16:22,500 we need to read that file. 203 00:16:26,230 --> 00:16:33,400 And as you can see there the last line of that log file is what we sent to it with the logger command. 204 00:16:33,400 --> 00:16:39,940 The format of these Assis log messages is a time stamp followed by the computer name followed by the 205 00:16:39,940 --> 00:16:44,330 tag which is again most often the name of the program writing to this log. 206 00:16:44,350 --> 00:16:47,640 In this case it's our username since we didn't specify a tag. 207 00:16:47,830 --> 00:16:49,930 And finally the message itself. 208 00:16:49,930 --> 00:16:55,540 So this time let's add a tag will do logger Dashti my script. 209 00:16:55,930 --> 00:16:59,850 Tagging on. 210 00:16:59,860 --> 00:17:05,360 So this time you can see that the tag reads my script and then we see the message that we passed into 211 00:17:05,390 --> 00:17:06,040 that. 212 00:17:06,190 --> 00:17:12,250 By the way since law can be configured to send messages off the server and to a centralized assist log 213 00:17:12,250 --> 00:17:13,940 location for example. 214 00:17:13,990 --> 00:17:19,000 Now I cover the details of exactly how to do this in my Linux in the real world course. 215 00:17:19,040 --> 00:17:24,660 Anyway that is a good security measure because if someone were to have root access to one system in 216 00:17:24,690 --> 00:17:30,490 the logs that they generated are not only stored locally where they could potentially change them or 217 00:17:30,490 --> 00:17:36,430 delete them but they're also stored on a remote system where hopefully that person or that attacker 218 00:17:36,430 --> 00:17:38,900 hasn't broken into that system as well. 219 00:17:39,100 --> 00:17:44,350 It's just something to keep in mind it would be a reason to use the standard syslog system built into 220 00:17:44,350 --> 00:17:50,220 Linux instead of for example creating your own logging service system from scratch. 221 00:17:50,720 --> 00:17:54,180 OK let's add this longer functionality to our script 222 00:18:00,440 --> 00:18:03,380 logger Dashti give it the name of our script. 223 00:18:06,850 --> 00:18:11,740 And then we'll send the message to the logfile. 224 00:18:11,870 --> 00:18:17,930 Now regardless of whether the message is displayed on standard out it will get recorded in the system 225 00:18:17,930 --> 00:18:18,260 log. 226 00:18:18,260 --> 00:18:19,590 So let's try it out here. 227 00:18:20,850 --> 00:18:24,420 When we execute our script our messages are displayed to the screen. 228 00:18:24,420 --> 00:18:27,020 Now let's see if they made it to the log file. 229 00:18:30,570 --> 00:18:36,660 And sure enough they have no it's turned verbose mode off and see what that's like. 230 00:18:36,950 --> 00:18:43,550 OK let's change this from true to false and execute our script. 231 00:18:43,630 --> 00:18:47,850 So there's no output to standard output there's nothing on our screen. 232 00:18:47,860 --> 00:18:51,580 Let's see if it made its way to the log file however 233 00:18:55,450 --> 00:19:00,230 Kay says hello and this is fun so let's do that again here and then. 234 00:19:00,310 --> 00:19:00,640 Yup. 235 00:19:00,700 --> 00:19:04,780 It keeps adding to the log file so that is working. 236 00:19:04,780 --> 00:19:07,900 Now let's go ahead and turn verbose mode back on 237 00:19:17,480 --> 00:19:19,490 before we write our next function. 238 00:19:19,490 --> 00:19:24,510 Let's add a quick comment to the top of this function explaining what it does. 239 00:19:24,530 --> 00:19:28,960 Now we do this for our scripts and so our functions are like little scripts within a script. 240 00:19:29,000 --> 00:19:31,760 So let's do that here to. 241 00:19:31,790 --> 00:19:33,540 So I'll go here and say 242 00:19:36,720 --> 00:19:47,250 this function sends a message to this log and to standard output if verbose is true. 243 00:19:47,600 --> 00:19:53,400 Hey if you wanted to add more information you can do that. 244 00:19:53,400 --> 00:19:58,680 Some people like to include things like any global variables that are used inside the function or by 245 00:19:58,680 --> 00:19:59,460 the function. 246 00:19:59,640 --> 00:20:04,470 Any arguments that the function accepts and anything the function might return. 247 00:20:04,470 --> 00:20:08,180 Now we haven't covered returns yet but we'll get to that soon enough. 248 00:20:08,190 --> 00:20:14,430 Moving on to our next to function as a reminder functions have to be defined before they are used. 249 00:20:14,460 --> 00:20:18,150 So a good practice is to put all your functions at the top of your script. 250 00:20:18,940 --> 00:20:24,790 The only things I would consider putting before the functions are constant global variables and sourcing 251 00:20:24,790 --> 00:20:29,630 script libraries again do not call a function before it's defined. 252 00:20:29,950 --> 00:20:35,230 Let's say you have a script that is going to update several files. 253 00:20:35,290 --> 00:20:39,090 You've lived a life long enough to know that sometimes things go wrong. 254 00:20:39,250 --> 00:20:43,650 So you want to make a backup of every single file before you modify it. 255 00:20:43,660 --> 00:20:46,760 Or more accurately before your script modifies it. 256 00:20:46,900 --> 00:20:52,720 That way if you have an error in your script or something unexpected happens you have a good copy of 257 00:20:52,720 --> 00:20:54,450 each file to go back to. 258 00:20:54,460 --> 00:20:58,710 So let's call this particular function back up underscore file 259 00:21:05,190 --> 00:21:06,320 only to come in here. 260 00:21:15,790 --> 00:21:19,670 All right we'll get to this return statement here in just a second. 261 00:21:19,700 --> 00:21:35,230 We're going to use a variable in our function so we'll make that local. 262 00:21:35,360 --> 00:21:38,620 We'll do a quick check to see if that file got passed and exists. 263 00:21:47,980 --> 00:21:52,000 Again we're setting a variable so we're going to use the local keyword here 264 00:22:09,840 --> 00:22:12,590 looks like I miss my clothing parentheses here. 265 00:22:12,590 --> 00:22:18,020 I mean do that. 266 00:22:18,140 --> 00:22:21,350 You're probably already familiar with the temp directory. 267 00:22:21,350 --> 00:22:28,220 However here I am using forward slash var forward slash temp because the files in var temp will survive 268 00:22:28,220 --> 00:22:33,200 a reboot while those in slash temp are not guaranteed to survive a reboot. 269 00:22:33,200 --> 00:22:39,970 Typically the files in temp are cleared on boot and are also cleared more often on a running system. 270 00:22:39,980 --> 00:22:45,560 I think the default for Scinto 7 is that files and temp are deleted every 10 days while the files in 271 00:22:45,560 --> 00:22:48,410 var temp are deleted every 30 days. 272 00:22:48,410 --> 00:22:53,870 That said I want to make sure that if I break a file and then do something like reboot the system that 273 00:22:53,870 --> 00:22:55,280 I don't lose that file. 274 00:22:55,280 --> 00:23:02,070 Hence the reason for me choosing the location of forward slash var Ford slash temp. 275 00:23:02,120 --> 00:23:04,400 All right continuing with our variable here. 276 00:23:04,400 --> 00:23:09,950 As you know the base name command removes the path to the file and just leaves us with a file name and 277 00:23:09,950 --> 00:23:15,440 we don't care about its original path because we just going to put a copy of it in var temp anyway. 278 00:23:15,440 --> 00:23:20,360 Also here we're using command substitution with the date command and you've used the date command with 279 00:23:20,360 --> 00:23:22,160 a couple of different formats before. 280 00:23:22,280 --> 00:23:28,790 But now I'm introducing the capital F format now capital F you can think of is standing for a full date 281 00:23:28,990 --> 00:23:33,080 and it displays the year month day using numbers. 282 00:23:33,080 --> 00:23:39,830 I love using this format because it makes sorting by date very easy once we call this function a couple 283 00:23:39,830 --> 00:23:42,010 times you'll see exactly what I mean. 284 00:23:42,020 --> 00:23:48,940 Also I attempted to account for multiple backups of the same file on the same day by appending the nanoseconds 285 00:23:48,950 --> 00:23:55,100 when the date command was called that's represented by the percent sign followed by capital N.. 286 00:23:55,400 --> 00:24:00,830 We could have used a shell built in variable here to something like Dollar Sign random or dollar sign 287 00:24:00,830 --> 00:24:07,760 bash PID or even Dollar Sign Dollar Sign which expands to the PID or process id of the current running 288 00:24:07,760 --> 00:24:08,900 script. 289 00:24:08,900 --> 00:24:11,830 So now in this function let's call our other function here. 290 00:24:49,110 --> 00:24:53,880 So after we use the log function to record what we're going to do here or communicate our intentions 291 00:24:53,880 --> 00:24:58,180 here then we're going to use the C-p command to actually perform a copy. 292 00:24:58,200 --> 00:25:04,710 Now the dash option to the copy command is short for preserve and it preserves the files mode ownership 293 00:25:04,740 --> 00:25:06,230 and time stamps. 294 00:25:06,390 --> 00:25:12,110 If you don't use the dash option then the copy of the original file will have the current time stamp. 295 00:25:12,120 --> 00:25:17,790 I personally like to preserve the original time stamp on the copy just in case I need to put that copy 296 00:25:17,790 --> 00:25:20,370 back in place because it's a backup. 297 00:25:20,370 --> 00:25:25,890 The contents of that file haven't changed so in my opinion it makes sense to keep its time stamps the 298 00:25:25,890 --> 00:25:29,750 way they were instead of updating the time stamp to today. 299 00:25:29,910 --> 00:25:35,760 In the grand scheme of things this is really minor but it's just something to think about each function 300 00:25:35,760 --> 00:25:39,520 like each command returns an exit status by default. 301 00:25:39,570 --> 00:25:45,120 The function returns the exit status of the last command it executes in the case where the file that 302 00:25:45,120 --> 00:25:47,490 was passed to the function actually exists. 303 00:25:47,550 --> 00:25:54,090 We'll just let C-p return whatever exit status is appropriate and use that as the exit status for our 304 00:25:54,090 --> 00:25:55,110 function. 305 00:25:55,110 --> 00:26:01,410 If for some reason the CB command fails it will return a non-zero exit status and thus our backup file 306 00:26:01,410 --> 00:26:05,680 function will return that same non zero exit status. 307 00:26:24,330 --> 00:26:29,940 Here if the file doesn't exist that means we obviously can't make a copy of it a backup copy of it. 308 00:26:30,120 --> 00:26:44,190 So our function more or less fails and that's why we're going to return a non zero exit status. 309 00:26:44,190 --> 00:26:47,040 Now let's go ahead and call this function in our code here. 310 00:26:48,010 --> 00:26:59,390 A backup file let's back up the NC password file and lets see it in action. 311 00:26:59,490 --> 00:27:05,460 K we have the output displayed to our screen because we have verbose mode on and then we also see the 312 00:27:05,460 --> 00:27:11,820 output that was generated inside the backup file function which in turn call the log function to tell 313 00:27:11,820 --> 00:27:14,010 us about the backup that it made. 314 00:27:14,280 --> 00:27:18,580 So let's see if these are messages made it to the log file. 315 00:27:21,280 --> 00:27:22,410 Hey sure enough they have. 316 00:27:22,410 --> 00:27:28,290 Now we have a record of exactly what happened during that run of that script and exactly when it happened 317 00:27:28,620 --> 00:27:35,230 and all that good information there is a look at the file we copied here into var temp. 318 00:27:35,290 --> 00:27:40,620 You can see that the file that was in the log message here is this file here. 319 00:27:40,710 --> 00:27:45,690 Obviously I was playing around with this script yesterday doing some tests and so that's why we have 320 00:27:45,750 --> 00:27:48,320 these other password files there. 321 00:27:48,330 --> 00:27:56,400 Now let's look at ANSI password know and see what happens when we execute the copy command with out 322 00:27:56,430 --> 00:27:58,930 the dash options or do C.P ETSI. 323 00:27:59,100 --> 00:28:05,210 He is a W.D. and will copy that into var temp and we'll look at the contents of our temp. 324 00:28:05,220 --> 00:28:11,740 Now right now the date is January 12th and the date of that file is also January 12th. 325 00:28:11,860 --> 00:28:16,230 But if you look at its password it's really January 7th. 326 00:28:16,360 --> 00:28:18,850 It's the original time stamp on that file. 327 00:28:18,870 --> 00:28:22,000 So again that was the difference with the dash p option. 328 00:28:22,380 --> 00:28:24,080 OK we're getting really close to the end. 329 00:28:24,080 --> 00:28:25,750 Just one more little block of code. 330 00:28:25,820 --> 00:28:28,370 So let's jump back into our script. 331 00:28:28,680 --> 00:28:33,690 So as we discussed earlier a function has a return code or exit status just like any other command on 332 00:28:33,690 --> 00:28:34,690 a Linux system. 333 00:28:34,830 --> 00:28:52,970 Now let's check that status. 334 00:28:53,080 --> 00:28:56,560 So let's do it in the positive if the return code is zero. 335 00:28:56,560 --> 00:29:04,970 That means it's success so that we can do then is say hey the file backup succeeded. 336 00:29:05,150 --> 00:29:15,130 And then if it was not 0 then we'll say while backup failed and we'll exit the entire script there. 337 00:29:15,140 --> 00:29:20,480 So again if the backup function succeeds we're going to get that log file messages as file backup succeeded 338 00:29:20,480 --> 00:29:28,070 if it fails will log a file backup failed and exit the entire script itself with a non-zero exit status. 339 00:29:28,070 --> 00:29:34,730 Now by the way inside a function it's super important to use return and not exit if the exit command 340 00:29:34,730 --> 00:29:42,290 gets executed it exits the entire script no matter if that exit command is inside a function or not. 341 00:29:42,470 --> 00:29:43,280 So exit. 342 00:29:43,280 --> 00:29:46,040 Remember the exit is for the entire script. 343 00:29:46,070 --> 00:29:48,120 Return is just for the function. 344 00:29:48,120 --> 00:29:50,730 So now let's try this little extra code out 345 00:29:54,780 --> 00:29:56,820 right it looks like our backup succeeded. 346 00:29:56,820 --> 00:29:58,500 Let's look at the messages file. 347 00:30:00,960 --> 00:30:01,270 OK. 348 00:30:01,300 --> 00:30:08,140 Our file backup succeeded message ended up in the messages file to if we were to pass the backup file 349 00:30:08,140 --> 00:30:15,340 function in a file that didn't exist we would trigger an error or if the disk was full or if the file 350 00:30:15,340 --> 00:30:20,680 system was in a read only state when the backup was attempted it would also trigger an error state and 351 00:30:20,740 --> 00:30:22,210 exit the script. 352 00:30:22,300 --> 00:30:27,670 By the way this bit of code was just to show you that you can check the exit status of a function just 353 00:30:27,670 --> 00:30:29,870 like you can any other command. 354 00:30:29,890 --> 00:30:34,240 However in this particular case you would want to check if the backup succeeded or not every single 355 00:30:34,240 --> 00:30:35,710 time you performed a backup. 356 00:30:35,890 --> 00:30:42,070 And that means in practice you would most likely add this particular check or this bit of code inside 357 00:30:42,280 --> 00:30:44,490 the backup file function itself. 358 00:30:44,740 --> 00:30:49,990 If you wanted to over engineer this thing you could totally create three functions to do the same bit 359 00:30:49,990 --> 00:30:50,630 of work. 360 00:30:50,800 --> 00:30:52,510 One function would perform the backup. 361 00:30:52,510 --> 00:30:58,450 One would be use to check if the backup actually succeeded in a third function would be the one that 362 00:30:58,450 --> 00:31:03,250 you would actually call in your script which would in turn call the other two functions it would be 363 00:31:03,250 --> 00:31:08,820 like a wrapper around those functions and it would look something like this. 364 00:31:08,830 --> 00:31:15,770 So we'll just declare a function here backup file and let's say we call our other function that actually 365 00:31:15,770 --> 00:31:23,590 does the backup and we'll call it perform back up and we'll just pass that right along here. 366 00:31:25,970 --> 00:31:30,640 Then we want to check the backup. 367 00:31:30,690 --> 00:31:35,910 This is actually going to fail when I call it because those other functions don't exist right we just 368 00:31:35,910 --> 00:31:41,930 kind of made this up on the fly so I'll do this backup file and see password for example. 369 00:31:42,030 --> 00:31:46,500 And sure enough when we call it it executes to perform backup but that command isn't found because again 370 00:31:46,500 --> 00:31:47,470 it doesn't exist. 371 00:31:47,580 --> 00:31:50,760 And then it calls check back up again it doesn't exist. 372 00:31:50,760 --> 00:31:56,690 So anyway as you might have noticed you can define a function at the command line like I did right here 373 00:31:57,000 --> 00:31:59,410 and you really already know this spinel right. 374 00:31:59,550 --> 00:32:04,060 A shell script is just automating what you can do at the command line anyway. 375 00:32:04,320 --> 00:32:08,450 So let's let's look at this type mâché back up on score file. 376 00:32:08,460 --> 00:32:14,550 It tells us it has a function and it displays the code that is contained in that function or that function 377 00:32:14,550 --> 00:32:15,530 definition. 378 00:32:15,540 --> 00:32:20,830 This way you can write functions and have them write inside your current running shell. 379 00:32:20,850 --> 00:32:24,540 I feel like we've covered a lot about functions don't you. 380 00:32:24,540 --> 00:32:31,370 Before we wrap this up let's do a little refresher of the main points about working with functions in 381 00:32:31,380 --> 00:32:37,010 this lesson you learned about the concept of dry which stands for don't repeat yourself. 382 00:32:37,080 --> 00:32:41,880 If you find yourself writing the same bit of code in multiple places in your script it's a good sign 383 00:32:41,880 --> 00:32:43,780 that you should use a function. 384 00:32:43,890 --> 00:32:49,680 You also learned that by default all variables are global in scope and you learn how to use the local 385 00:32:49,680 --> 00:32:55,110 keyword to define local variables that are only available within the function that they are defined 386 00:32:55,110 --> 00:32:56,140 in. 387 00:32:56,160 --> 00:32:59,590 You also learned how to use positional parameters within your functions. 388 00:32:59,790 --> 00:33:06,060 And finally we covered how to use EXIT statuses with your functions by using the return command.